Show that the area of a rhombus is half the product of the lengths of its diagonals.

What is the area of this trapezoidal garden (All measurements are in cm)? 

1. $60$ sq.cm
2. $180$ sq.cm
3. $210$ sq.cm 
4. $240$ sq.cm

The three sides of a triangle are $15 cm, 36 cm, and 39 cm$ long. A small triangle is formed by joining the mid points of the three sides of this triangle. The area of small triangle will be_____ $c{m}^2$  

The length of rectangle is $80\%$ of diagonal of square of area $1225 {\mathrm{m}}^2$, then find area of rectangle, if it's perimeter is $94\sqrt{2} \mathrm{m}$.

In $△ABC, AD$ is the median and $G$ is a point on $AD$ such that $AG:GD=2:1$. Then area $(△BDG):$area $(△ABC)$ is equal to: